A voltage-controlled current source, or a transconductor, is an important building block in electronic circuits. Transistors, based on either the bipolar junction or field effect principle, essentially perform such a function. The voltage-to-current, or V-I, characteristics of transistors, however, are usually not sufficiently linear for large-signal applications such as a transmitting modulator. Linear impedances are therefore often combined with transistors to form linear transconductors. FIG. 1a shows a bipolar transistor with resistive emitter degeneration as a linear transconductor. When the product of the transistor's transconductance gm and the degenerating resistance R is much greater than unity the overall ratio between the output current Io and the input voltage Vin is then given by 1/R, which is linear. Another example is given in FIG. 1b, where the combination of transistor T and amplifier OP makes the effective input impedance of the structure (seen from node 1) much smaller than the linear impedance Z, so that the output current Io is related to the control voltage Vin and the bias current IB by either Io=IB+(Vin−VB)/Z, if the said control voltage is applied to node 2 or Io=IB−(Vin−VB)/Z, if the said control voltage is applied to node 3. In such and similar schemes the linear impedance Z is connected to the low impedance node of the transistor, that is, the emitter for the bipolar junction transistor or the source for the field effect transistor. This usually makes it necessary to bias the said emitter or source terminal of the current-source transistor away from a common reference point such as the ground terminal GND or a power supply VDD, as the case may be, to allow voltage headroom for the necessary supporting circuitry. This headroom reduces the usable voltage swing at the output node (the collector or drain terminal of the transistor) compared to a transistor in common-source or common-emitter configuration, which we refer to as a grounded transconductor. In modern integrated circuits where the permissible supply voltage is very limited, the loss of available signal swing to the bias requirement of the linearizing circuitry is becoming unacceptably large in relative terms. With the incorporation of linearizing resistor(s), bias current source(s) and operational amplifier, the achievable signal-to-noise ratio for the output current can also be seriously degraded by the inevitable noise sources associated with such additional elements, as compared to a grounded transconductor, an example of which is depicted in FIG. 1c. In transmitter applications noise from such linearized transconductors is usually the cause for typical noise performance of the modulating mixer being two orders of magnitude worse than that which is required by applications such as GSM. Polar modulators (where no modulating mixers are required) are therefore overwhelmingly preferred over Cartesian modulators for this reason, despite the ease with which the latter modulator type can accommodate both amplitude and phase modulation schemes as required by the latest mobile communications standards such as wireless LAN, EDGE and UMTS.